1. Field of the Invention
The present invention relates to an organic electroluminescent element in which a transparent electroconductive layer is formed on a light emitting layer by such as a sputtering method.
2. Description of the Related Art
The organic electroluminescent (which may be hereinafter abbreviated as EL) element in which the light emitting layer is sandwiched between a pair of electrodes and light is emitted by applying voltage between the electrodes has advantages such as follows: high visibility due to self light emission, excellent impact resistance because it is an entirely solid element unlike a liquid crystal element, a high response speed, less susceptibility to temperature changes, and a large viewing angle. Thus, the EL element is attracting attention for applications as light emitting elements in display devices.
The organic EL element fundamentally takes a laminated structure of an anode layer/a light emitting layer/a cathode layer. Conventionally, it is a general practice that the anode layer as a transparent electroconductive layer is laminated on a transparent substrate such as a glass substrate, and a light emitting layer and a cathode layer are subsequently laminated on the anode layer. Thus, a bottom emission type in which light is taken out of a side of the anode layer has been employed.
On the other hand, a top emission type in which a cathode layer is designed as a transparent electroconductive layer and light is taken out of a side of the cathode layer has recently been noted. With realization of the top emission type, if the cathode and the anode are designed as transparent electroconductive layers, an entirely transparent light emitting element can be obtained, which can realize light emission from both faces of the element. Since an arbitrary color can be adopted as a background color for such a transparent light emitting element, the element can make a display device which retains a colored state even at a time when it emits no light, and thereby enhance decorativeness. Further, since the emission of the light is not shielded in the top emission type by TFTs (thin film transistors) in case of an active driving display device, the display device having a high aperture ratio can be provided.
As an example of the top emission type organic EL element, an organic EL element is disclosed, in which an organic light emitting layer is formed between an anode layer and a cathode layer, the cathode layer is composed of an electron injecting layer and a transparent electroconductive layer, and the electron injecting layer is arranged on a side of the organic light emitting layer (Japanese Patent Application Laid-Open (JP-A) 10-1162959).
However, in the case of the top emission type organic EL element, a transparent electroconductive film layer made of ITO or the like is generally formed by a sputtering method. Therefore, when the transparent electroconductive layer is formed on the light emitting layer, members such as the light emitting layer, the electron injecting layer and the hole injecting layer are impacted with such as sputtered particles, Ar+ and ionized electrons at the time of the sputtering. Consequently, there was a problem that the light emitting characteristics are deteriorated (reduction in the current density, reduction in the light emitting efficiency, and the current leakage). In addition, when plasma is used to form the transparent electroconductive layer, there was a problem that the light emitting layer was exposed to a plasma atmosphere and the light emitting characteristics were likely to be deteriorated. Furthermore, there was another problem that when oxygen is introduced or oxygen is released from a target in forming the transparent electroconductive layer, there was a problem that a highly reactive metal contained in the electron injecting layer was oxidized, thereby deteriorating the characteristics of the electron injecting layer (reduction in the current density, reduction in the light emitting efficiency and the enlargement of dark spots).
In order to solve such problems, trials have been made to form various layers between the light emitting layer and the transparent electroconductive layer (upper electrode). For example, there are disclosed an organic EL element in which a sputtering protection layer made of gold, nickel or aluminum is formed between a light emitting layer and a transparent electroconductive layer (JP-A2003-77651); an organic EL element in which a cathode layer is designed as a two-layer structure and between first and second cathode layers is formed an electron transporting protecting layer made of an electron transporting organic material such as bathocuproin (BCP) doped with an alkali metal or alkaline earth metal (such as Li, Cs, Ba, Sr, Ca) (JP-A2004-127740); an organic EL element in which a cathode layer has a thin metal layer and this thin metal layer is covered with a protective layer made of a semiconductor (ZnSe or the like) having a wide band gap (JP-A10-223377); an organic EL element in which a hole injecting layer having a thickness of 30 to 1000 nm is formed between an organic light emitting layer and an upper transparent anode layer (JP-A2004-227943); and an organic EL element in which between an organic EL layer and a transparent electroconductive layer is formed a buffer layer made of a phthalocyanine compound doped with a metal such as Au, Pt or Ag (JP-A2004-296234).
However, when various layers are inserted between the light emitting layer and the transparent electroconductive layer (upper electrode) as mentioned above, obviously, transparency are required for those layers. Unless the electron transporting property and the electron injecting property of those layers are sufficiently high, there were problems that the electric resistance of the entire element became higher, that the current-voltage characteristic was deteriorated, and that balance between the holes and the electrons injected into the light emitting layer was disrupted, so that the light emitting characteristics were deteriorated.